1. Field of the Invention
The present invention relates to a steering-heater device for heating a steering wheel of a vehicle. More particularly, it relates to a steering-heater device which is capable of controlling the temperature of a heater without causing an excess current to flow into the steering wheel.
2. Description of the Related Art
Japanese Examined Patent Publication (kokoku) No. 2-30910 discloses a steering wheel whose ring part is equipped with a heating member.
In this steering wheel, there is arranged, in the vicinity of the heating member of the steering wheel, a thermistor serving as a thermo-sensitive element. In operation, the thermistor detects a temperature change of the heating member in order to carry out the temperature control for the steering wheel.
On the higher-temperature side of the heating member in the steering wheel, there is provided a fuse which is melted and broken to stop the supply of current when the temperature of the heating member rises excessively.
In the above-mentioned steering wheel, however, when a heater wire in the heating member shorts, an excess current continues to flow into the steering wheel until the fuse is melted.
In the above way, if the excess current flows into the steering wheel until the fuse is melted, then a problem arises in a possibility that a flexible flat cable (FFC) connecting the steering wheel with a steering column has an abnormality in function because the FFC has a low allowable current value.
Under such a situation, it is therefore an object of the present invention to provide a steering-heater device which is capable of controlling the temperature of a steering wheel without causing an excess current to flow into the steering wheel.
The above-mentioned object can be solved by providing a steering-heater device for a vehicle, comprising:
a heater disposed in a steering wheel of a vehicle;
a calorific switching element connected to the heater electrically and arranged in a steering column to switch the flowing of a drive current for driving the heater, the calorific switching element generating heat by the drive current;
a thermo-sensitive element arranged adjacent to the calorific switching element to sense a temperature of the calorific switching element thereby to change characteristics of the thermo-sensitive element;
a control unit connected to the thermo-sensitive element to detect an initial temperature of the steering wheel on a basis of a change in the characteristics of the thermo-sensitive element, the control unit further calculating a consumption power necessary to rise a temperature of the steering wheel from the initial temperature to a predetermined temperature and also outputting a control signal for a heating period determined on a basis of the consumption power; and
a drive unit connected to the control unit to output a drive signal to drive the calorific switching element on a basis of the control signal outputted from the control unit.
According to the steering-heater device constructed above, as the calorific switching element in the steering column allows of blocking of an excess current, it is possible to prevent the excess (current from flowing into the steering wheel, thereby excluding a possibility that a connecting cable (e.g. FFC) between the steering wheel and the steering column has an abnormality in its connecting function.
As the second aspect of the invention, the above steering-heater device further comprises a drive-stop unit which detects a voltage drop of the calorific switching element due to the flowing of the drive current and makes the driving of the calorific switching element stop when no voltage drop is detected.
According to the second aspect of the invention, it is possible to prevent the calorific switching element from being still driven despite that an operational route for the heater has been opened.
As the third aspect of the invention, the calorific switching element of the steering-heater device is a power MOSFET where a voltage between gate and source thereof is established so that the drive current becomes less than a current limit for the heater.
According to the third aspect of the invention, owing to the above establishment of a voltage between gate and source of the power MOSFET, it is possible to prevent the excess current from flowing into the steering wheel, thereby excluding a possibility that the connecting cable (e.g. FFC) between the steering wheel and the steering column has an abnormality in its connecting function.
As the fourth aspect of the invention, the steering-heater device further comprises a voltage detecting unit connected to the thermo-sensitive element and the control unit thereby to detect a voltage between both ends of the thermo-sensitive element. Then, the characteristics of the thermo-sensitive element is identical to the voltage between both ends of the thermo-sensitive element.
As the fifth aspect of the invention, in the steering-heater device of the fourth aspect, the thermo-sensitive element is a thermo-sensitive diode through which a constant current flows and which senses the temperature of the power MOSFET thereby to change the voltage between both ends of the thermo-sensitive diode.
As the sixth aspect of the invention, in the steering-heater device of the fifth aspect, the power MOSFET and the thermo-sensitive element are formed into one body thereby to provide a power semiconductor having a built-in diode.
According to the sixth aspect of the invention, owing to the integration of the power MOSFET with the thermo-sensitive element, it is possible to simplify the structure inside the steering column.
As the seventh aspect of the invention, the heater is connected to the calorific switching element through a flexible flat cable.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims taken in conjunction with the accompany drawing.